It is a well-known fact that mercury is necessary in manufacturing practically all kinds of gas-discharge lamps, e.g. mercury rectifiers, lasers, and fluorescent tubes. Fluorescent tubes are made as glass tubes whose internal surfaces are covered with fluorescent materials. The tubes are filled up with an inert gas, e.g. argon or neon, together with a minimal amount of mercury vapor. Mercury constitutes the major ingredient that ensures functioning of such tubes. However, high toxicity of mercury creates serious environmental problems both in the manufacture of such tubes and in dismantling of mercury-containing devices either upon expiration of their service life or in case of a failure thereof.
Ever increasing annual production, as well as steady growth of the range of tubes produced e.g. for such sector as neon industry, have required to impose certain restrictions on mercury application, and to establish minimum admissible amounts of mercury, compatible with the requirement to the functioning of such tubes. At present, a legal framework is being prepared in many countries with the aim of introducing international regulations on the use of mercury, establishing minimum admissible amounts of mercury for each product.
In the past, a rather widespread method consisted in introducing mercury through an exhaust tube into the working area of a tube. Open use of liquid mercury however entails a number of problems. Firstly, certain complexities during liquid mercury storage and transportation are inevitable due to a high pressure of its vapor at room temperature. Secondly, the main disadvantage of introducing liquid mercury prior to vacuum treatment of a lamp (tube) consists in the ingress of mercury into the equipment in the process of evacuation, thereby resulting in release of vapor into the environment. Thirdly, in case of introducing liquid mercury, its precise dosing is practically impossible. Generally, mercury is supplied into a lamp in considerably bigger amounts than calculated ones. This is caused by the fact that due to indirect heating, the liquid mercury contained within the lamp is exposed to surface oxidation and gets combined with materials from which an electrode is made; therefore, in practice only about 40% of the total amount of mercury introduced into the lamp may be used. In other words, in addition to environmental problems, such method results in an excessive consumption of mercury and does not permit to ensure accurate and reproducible dosing.
With the aim of overcoming the above drawbacks, prior art solutions have offered alternative technologies based e.g. on the use of soldered capsules containing certain amounts of liquid mercury.
Such capsules, being preferably of cylindrical shape, were mounted within the working space of a tube, following which evacuation was carried out, and mercury was released into the internal volume of the tube e.g. due to thermal effect.
Thus, U.S. Pat. No. 4,182,971 discloses the use of glass capsules that contain mercury and are fastened on an auxiliary electrode inside the tube working area. A capsule was heated by high-frequency effect, thereby resulting in glass cracking and hence the release of mercury vapor. Due to the long duration of the heating effect, such technology inevitably results in mercury oxidation and therefore its excessive consumption. In addition, implementation of this known method requires special electrodes of a rather complicated design.
To prevent possible complete destruction of a capsule and ingress of glass fragments into the internal space of a lamp, U.S. Pat. No. 4,335,326 proposes to place the capsule within the tube, inside a protective shield made of glass or metal. It is obvious that mounting of such capsules within the working area is rather complicated, and the process of operation does not exclude a damage to the internal structure of the tube.